Boundary Scavenging Research Articles

230Th and 231Pa on GEOTRACES GA03, the U.S. GEOTRACES North Atlantic transect, and implications for modern and paleoceanographic chemical fluxes

The long-lived uranium decay products 230Th and 231Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of 230Th and 231Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using 230Th-normalization and to demonstrate enhanced 231Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of 231Pa is apparent. The 231Pa/230Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation.

Biogeography in 231Pa/230Th ratios and a balanced 231Pa budget for the Pacific Ocean

The ratio of unsupported protactinium-231 to thorium-230 in marine sediments, (Pa/Th)xs, is potentially sensitive to several processes of oceanographic and climatological interest: deep ocean circulation, marine biological productivity (as it relates to total particle flux) and particle composition (specifically, biogenic opal and authigenic Mn). In order to attribute variations in (Pa/Th)xs observed in sediment records to changes in specific processes through time, a better understanding of the chemical cycling of these elements in the modern ocean is necessary. To this end, a survey was undertaken of (Pa/Th)xs in surface sediments from the subarctic Pacific (SO202-INOPEX expedition) in combination with a Pacific-wide compilation of published data. Throughout the Pacific, (Pa/Th)xs is robustly correlated with the opal content of sediments. In the North and equatorial Pacific, simultaneous positive correlations with productivity indicators suggest that boundary scavenging and opal scavenging combine to enhance the removal of Pa in the eastern equatorial Pacific and subarctic Pacific. Deep ocean water mass ageing (>3.5 km) associated with the Pacific overturning appears to play a secondary role in determining the basin scale distribution of (Pa/Th)xs. A basin-wide extrapolation of Pa removal is performed which suggests that the Pacific Pa budget is nearly in balance. We hypothesize that through time (Pa/Th)xs distributions in the Pacific could define the evolving boundaries of contrasting biogeographic provinces in the North Pacific, while the influence of hydrothermal scavenging of Pa potentially confounds this approach in the South Pacific.

Sedimentation and lateral transport of 210Pb over the East China Sea Shelf

210Pb is an effective tracer of constraining particle transport and sedimentation in shelf regions. To reveal the spatial pattern of 210Pb over the East China Sea (i.e. ECS) Shelf, 210Pb in the surface sediments were examined at 11 stations, as well as 234Th and 210Pb in the water column at four stations. Overall, the plume zone of the Yangtze River along the coastline is a source area of 210Pb for the outer shelf, exporting 0.46 dpm cm−2 year−1 at least, which accounts for about 25 % of 210Pb input into this region. In the southern ECS Shelf to the north of the Taiwan Strait, the focusing factor (f) values are higher than unity, indicating a sink area of 210Pb. Boundary scavenging of 210Pb contributes 0.36 dpm cm−2 year−1 to this sink area on the basis of a mass balance model evaluation. Lateral transport of 210Pb to this region, quantified by 234Th and 210Pb in the water column, varied from 3.34 to 6.39 dpm cm−2 year−1 with an average of 4.83 dpm cm−2 year−1, also supporting its sink characteristic. To the southwest of the Cheju Island, the f values were less than unity, revealing a source region of 210Pb. The average export flux of 210Pb from this region was 1.64 dpm cm−2 year−1. Therefore, 210Pb sedimentation/settling showed significantly heterogeneous sedimentation of particulate matter over the ECS Shelf.

210Pb and 137Cs in margin sediments of the Arctic Ocean: Controls on boundary scavenging

210Pb and 137Cs were measured in 25 sediment cores collected during the International Polar Year from a transect spanning the North American Arctic margin, from the North Bering Sea to Baffin Bay/Davis Strait. Profiles and inventories of the radioisotopes were used to determine sediment mixing and accumulation at each site and to assess the intensity of scavenging and burial. Sediment accumulation rates derived from 210Pb and validated using 137Cs are between ≤0.04 and 0.23 g cm−2 yr−1. 210Pb cannot be used to derive sedimentation rates for vigorously biomixed sediments from the North Bering‐Chukchi shelf. Elevated 137Cs activities and inventories in recently deposited sediments imply delayed inputs of particle‐associated 137Cs to the sediments, likely transported from the watershed to the coast and subsequently redistributed to shelf/slope sediments. Inventories of 210Pbex in all cores meet or exceed the estimated supply of 210Pbex from atmospheric deposition and decay of 226Ra in the water column. This implies that in contrast to the deep Arctic Ocean basin, there is a sufficient supply of suspended particulates along the North American Arctic margin to scavenge the supply of 210Pbex. 210Pbex inventories in sediments are up to 21‐fold greater than the in situ supply at some sites. Large inventories of 210Pbex in sediments along the North Bering‐Chukchi shelf result primarily from focusing, while those along the north Chukchi slope (Barrow Canyon) and in Baffin Bay/Davis Strait reflect strong boundary scavenging, likely supported by lateral exchanges with deep/interior Atlantic‐origin waters.

A new perspective on boundary scavenging in the North Pacific Ocean

Boundary scavenging, or the enhanced removal of adsorption-prone elements from the ocean in areas of high particle flux, is an often cited, though not well-quantified, concept used to understand the oceanic distribution of many trace metals. Because 230Th and 231Pa are produced uniformly from uranium decay and removed differentially by scavenging, the process of boundary scavenging can be elucidated by a more detailed knowledge of their water column distributions. To this end, filtered seawater was collected across the gradients in particle flux which span the subarctic Pacific: in the west during the Innovative North Pacific Experiment (INOPEX) and in the east along Line P. Lateral concentration gradients of dissolved 230Th are small throughout the subarctic Pacific at 12 sites of variable particle flux. This contradicts the prediction of the traditional boundary scavenging model. A compilation of water column data from throughout the North Pacific reveals much larger lateral concentration gradients for 230Th between the subarctic North Pacific and subtropical gyre, over lateral gradients in scavenging intensity similar to those found within the subarctic. This reflects a biogeochemical-province aspect to scavenging. Upper water column distributions of 231Pa and 231Pa/230Th ratio are consistent with the influence of scavenging by biogenic opal, while deep waters (>2.5km) reveal an additional 231Pa sink possibly related to manganese oxides produced at continental margins or ridge crests.

Boundary scavenging at the East Atlantic margin does not negate use of 231Pa/230Th to trace Atlantic overturning

The 231Pa/230Th method is a promising tool to reconstruct Ocean circulation over the past Glacial–Interglacial cycle. However, marine particle flux may constrain the applicability of this ratio as a direct quantitative proxy for the strength of Atlantic Meridional Overturning Circulation (AMOC) by influencing the fractionation between the in situ produced 231Pa and 230Th in ocean water. Here we present 231Pa/230Th down-core profiles from high particle flux areas off Namibia and Senegal covering the past ∼35ka. The 231Pa/230Th profiles at these sites show very different responses to temporal variations of particle fluxes and to changes in water masses. Our results show that sedimentary 231Pa/230Th in the Eastern Atlantic margin is linked to particle flux, but controlled primarily by the mode of the AMOC. Our data suggest that during the past ∼30ka the high productivity Eastern margin was not capable of importing and storing significant amounts of 231Pa from the open Ocean. Consequently, the applicability of the 231Pa/230Th proxy to reconstruct past ocean circulation is not hampered by this potentially additional 231Pa sink.

Aluminium in an ocean general circulation model compared with the West Atlantic Geotraces cruises

A model of aluminium has been developed and implemented in an Ocean General Circulation Model (NEMO-PISCES). In the model, aluminium enters the ocean by means of dust deposition. The internal oceanic processes are described by advection, mixing and reversible scavenging. The model has been evaluated against a number of selected high-quality datasets covering much of the world ocean, especially those from the West Atlantic Geotraces cruises of 2010 and 2011. Generally, the model results are in fair agreement with the observations. However, the model does not describe well the vertical distribution of dissolved Al in the North Atlantic Ocean. The model may require changes in the physical forcing and the vertical dependence of the sinking velocity of biogenic silica to account for other discrepancies. To explore the model behaviour, sensitivity experiments have been performed, in which we changed the key parameters of the scavenging process as well as the input of aluminium into the ocean. This resulted in a better understanding of aluminium in the ocean, and it is now clear which parameter has what effect on the dissolved aluminium distribution and which processes might be missing in the model, among which boundary scavenging and biological incorporation of aluminium into diatoms.

Influence of intense scavenging on Pa-Th fractionation in the wake of Kerguelen Island (Southern Ocean)

Abstract. Dissolved and particulate excess 230Th and 231Pa concentrations (noted 230Thxs and 231Paxs respectively) and 231Paxs/230Thxs activity ratios were investigated on and out of the Kerguelen plateau (Southern Ocean) in the framework of the Kerguelen Ocean and Plateau compared Study project in order to better understand the influence of particle flux and particle chemistry and advection on the scavenging of 231Pa. In the wake of Kerguelen, particulate 231Paxs is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between 230Th and 231Pa (FTh/Pa ranging from 0.06 ± 0.01 to 1.6 ± 0.2) reflects the domination of the biogenic silica in the particle pool. Along the eastern escarpment of the Kerguelen plateau, the strong 231Paxs horizontal gradient in the deep waters highlights the intense removal of 231Pa at depth, as already observed for 230Thxs. This local boundary scavenging was attributed to re-suspension of opal-rich particles by nepheloid layers, resulting in fractionation factors FTh/Pa ≤ 1 along the Kerguelen plateau slope. Therefore, both the composition (biogenic opal) and the flux (intense along the margin) of particles control the scavenging of the two radionuclides in the Kerguelen wake. The modelling of 231Pa distribution with an advection-scavenging model demonstrates that lateral advection of open ocean water on the Kerguelen plateau could supply most of the 231Pa, which is then efficiently scavenged on the highly productive plateau, as previously proposed for 230Thxs. It stresses that lateral advection can play a significant role in the overall budget of particle reactive trace elements in a coastal-open ocean system.

The negligible role of intermediate water circulation in stadial–interstadial oxygenation variations along the southern California margin: Evidence from Nd isotopes

Changes in the source of intermediate waters to the southern California margin may have caused variations in seafloor oxygen levels on stadial–interstadial time scales. We test this hypothesis using the Nd isotopic composition of benthic foraminifera and fossil fish debris from ODP Sites 893 and 1017 to track the composition of intermediate waters across interstadials 8–14 (∼37–52 ka) during Marine Isotope Stage 3. The ɛ Nd values of waters bathing the seafloor at Site 893 were typically ∼−9 and those bathing Site 1017 were ∼−7, both of which are significantly less radiogenic than waters that had originated in either the North Pacific or Southern Ocean (by the time such waters reached the southern California margin). Detrital silicate ɛ Nd values of nearly −12 suggest that this offset toward lower ɛ Nd values was likely caused by boundary scavenging that partially overprinted the water mass composition with local/regional fluvial Nd inputs. In spite of the evidence for boundary scavenging, the lack of systematic seawater Nd isotope changes on a stadial–interstadial basis suggests that the provenance of the intermediate waters did not change, and that the waters were derived from the Southern Ocean. Instead, changes in local/regional sea surface productivity may have caused the recorded changes in seafloor oxygenation.

Modelling the effect of boundary scavenging on Thorium and Protactinium profiles in the ocean

Abstract. The "boundary scavenging" box model is a cornerstone of our understanding of the particle-reactive radionuclide fluxes between the open ocean and the ocean margins. However, it does not describe the radionuclide profiles in the water column. Here, I present the transport-reaction equations for radionuclides transported vertically by reversible scavenging on settling particles and laterally by horizontal currents between the margin and the open ocean. Analytical solutions of these equations are compared with existing data. In the Pacific Ocean, the model produces "almost" linear 230Th profiles (as observed in the data) despite lateral transport. However, omitting lateral transport biaises the 230Th based particle flux estimates by as much as 50%. 231Pa profiles are well reproduced in the whole water column of the Pacific Margin and from the surface down to 3000 m in the Pacific subtropical gyre. Enhanced bottom scavenging or inflow of 231Pa-poor equatorial water may account for the model-data discrepancy below 3000 m. The lithogenic 232Th is modelled using the same transport parameters as 230Th but a different source function. The main source of the 232Th scavenged in the open Pacific is advection from the ocean margin, whereas a net flux of 230Th produced in the open Pacific is advected and scavenged at the margin, illustrating boundary exchange. In the Arctic Ocean, the model reproduces 230Th measured profiles that the uni-dimensional scavenging model or the scavenging-ventilation model failed to explain. Moreover, if lateral transport is ignored, the 230Th based particle settling speed may by underestimated by a factor 4 at the Arctic Ocean margin. The very low scavenging rate in the open Arctic Ocean combined with the enhanced scavenging at the margin accounts for the lack of high 231Pa/230Th ratio in arctic sediments.

Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model

Abstract. The decoupled behaviour observed between Nd isotopic composition (Nd IC, also referred as εNd) and Nd concentration cycles has led to the notion of a "Nd paradox". While εNd behaves in a quasi-conservative way in the open ocean, leading to its broad use as a water-mass tracer, Nd concentration displays vertical profiles that increase with depth, together with a deep-water enrichment along the global thermohaline circulation. This non-conservative behaviour is typical of nutrients affected by scavenging in surface waters and remineralisation at depth. In addition, recent studies suggest the only way to reconcile both concentration and Nd IC oceanic budgets, is to invoke a "Boundary Exchange" process (BE, defined as the co-occurrence of transfer of elements from the margin to the sea with removal of elements from the sea by Boundary Scavenging) as a source-sink term. However, these studies do not simulate the input/output fluxes of Nd to the ocean, and therefore prevents from crucial information that limits our understanding of Nd decoupling. To investigate this paradox on a global scale, this study uses for the first time a fully prognostic coupled dynamical/biogeochemical model with an explicit representation of Nd sources and sinks to simulate the Nd oceanic cycle. Sources considered include dissolved river fluxes, atmospheric dusts and margin sediment re-dissolution. Sinks are scavenging by settling particles. This model simulates the global features of the Nd oceanic cycle well, and produces a realistic distribution of Nd concentration (correct order of magnitude, increase with depth and along the conveyor belt, 65% of the simulated values fit in the ±10 pmol/kg envelop when compared to the data) and isotopic composition (inter-basin gradient, characterization of the main water-masses, more than 70% of the simulated values fit in the ±3 εNd envelop when compared to the data), though a slight overestimation of Nd concentrations in the deep Pacific Ocean may reveal an underestimation of the particle fields by the biogeochemical model. Our results indicate 1) vertical cycling (scavenging/remineralisation) is absolutely necessary to simulate both concentration and εNd, and 2) BE is the dominant Nd source to the ocean. The estimated BE flux (1.1×1010 g(Nd)/yr) is much higher than both dissolved river discharge (2.6×108 g(Nd)/yr) and atmospheric inputs (1.0×108 g(Nd)/yr) that both play negligible role in the water column but are necessary to reconcile Nd IC in surface and subsurface waters. This leads to a new calculated residence time of 360 yrs for Nd in the ocean. The BE flux requires the dissolution of 3 to 5% of the annual flux of continental weathering deposited via the solid river discharge to the continental margin.

Modelling the effect of boundary scavenging on Thorium and Protactinium profiles in the ocean

Modelling the effect of boundary scavenging on Thorium and Protactinium profiles in the ocean

Particle dynamics study in the wake of Kerguelen Island using thorium isotopes

In the context of the KErguelen Ocean and Plateau compared Study (KEOPS, 19 January–13 February 2005), particle dynamics were investigated using thorium isotope measurements over and off the Kerguelen plateau. Dissolved and particulate 230Th and 232Th samples were collected at nine stations. Dissolved excess 230Th concentrations ( 230Th xs) vary from 0.5 to 20.8 fg/kg and particulate 230Th xs concentrations from 0.1 to 10.0 fg/kg. Dissolved and particulate 232Th concentration ranges are 16.8–450.2 pg/kg and 3.8–502.8 pg/kg, respectively. The 230Th xs concentrations increase linearly with depth down to the bottom at most of the plateau stations and down to 1000 m at the off-plateau stations. This linear trend is observed down to the bottom (1550 m) at Kerfix, the open-ocean “upstream” station located west of the Kerguelen plateau. A simple reversible scavenging model applied to these data allowed the estimation of adsorption rate constant ( k 1≈0.2–0.8 year −1), desorption rate constant ( k −1≈1–8 year −1) and partition coefficients (average K=0.16±0.07). Calculated particle settling velocities S deduced from this simple model are ca. 500 m/year at most of the plateau stations and 800 m/year at all the off-plateau stations. The plateau settling velocities are relatively low for such a productive site, compared to the surrounding HNLC areas. The difference might reflect the fact that lateral advection is neglected in this model. Taking this advection into account allows the reconstruction of the observed 230Th xs linear distributions, but only if faster settling velocities are considered. This implies that the 1D model strongly underestimates the settling velocity of the particles. In the deep layers, the occurrence of intense boundary scavenging along the escarpment due to bottom sediment re-suspension and interaction with a nepheloid layer, yielding a removal of ∼50% of the Th stock along the northwestward transect, is suggested.

Advection and scavenging: Effects on 230Th and 231Pa distribution off Southwest Africa

We investigated the controls of hydrography and of scavenging on the distribution of the particle reactive radionuclides 231Pa and 230Th in the water column and in surface sediments off Southwest Africa (Angola and Cape basins). Based on a vertical section of total 230Th ex concentrations in the water column we show that small differences in the salinity between the North Atlantic Deep Water (NADW) in the Angola Basin and the NADW in the Cape Basin as well as the advection of NADW associated with the Namib Col Current are reflected in total 230Th ex concentrations. These variable total concentrations are believed to reflect the flow path and mixing history of NADW with the NADW in the Angola Basin being relatively older and 230Th enriched compared to the NADW in the Cape Basin. In the area investigated we found high 231Pa ex/ 230Th ex ratios ( 231Pa ex/ 230Th ex > 0.093) in surface sediments at the continental margin and lower ones ( 231Pa ex/ 230Th ex < 0.093) in the open ocean. Such a distribution is normally interpreted to result from high particle flux at ocean margins (boundary scavenging). However, the lack of any significant depletion of dissolved 230Th and 231Pa in the water column does not indicate extensive scavenging at the continental margin. High 231Pa ex/ 230Th ex ratios are constrained to shallow waters depths (< 2000 m) only and coincide with low fractionation between 231Pa and 230Th indicating that preferential scavenging of 231Pa on opal may have caused high 231Pa ex/ 230Th ex ratios in the sediments. The observed close negative correlation ( r 2 = 0.82) between 231Pa ex/ 230Th ex ratios in sediments and water depths is believed to reflect changes in the particle composition, i.e. a decrease in opal content with water depth. In the Angola and Cape basins the total 231Pa ex concentrations in NADW were the highest observed so far in the Atlantic Ocean, and they are attributed to the meridional export of 231Pa from the North Atlantic. This caused the average dissolved 231Pa/ 230Th in the Southeast Atlantic to be about a factor 2 higher when compared to the North Atlantic (Labrador Sea). These differences in the dissolved 231Pa/ 230Th were not reflected in 231Pa/ 230Th ratios of surface sediments because the fractionation is lower in the Labrador Sea compared to the Southeast Atlantic, i.e. fractionation counteracts changes in the dissolved 231Pa/ 230Th. This suggests that fractionation is more important for the determination of 231Pa ex/ 230Th ex ratios in sediments than the meridional export of 231Pa from the North Atlantic.

Is there a 230Th deficit in Arctic sediments?

In most of the global ocean, the radionuclide thorium-230 is removed from the water column by adsorption onto particles and deposition in seafloor sediments, at a rate approximately in balance with its local production by the decay of uranium dissolved in seawater, allowing its use in assessing rates of marine processes. However, several previous studies have suggested that the flux of 230Th to the sediments of the Central Arctic is far too small to balance its production in the overlying water column. If this is so, 230Th produced in the low particle-flux Central Arctic basins would be deposited elsewhere, either by boundary scavenging at the margins or by export from the Arctic to lower latitudes. In order to evaluate this possibility, we compare the expected 230Th production and measured inventories for five sites in the Western Arctic, combining previously published 230Th data with reported AMS radiocarbon dates, and find no evidence for a substantial deficit of 230Th in these sediments. Instead, we find evidence for near balance in the 230Th budget during both the Holocene and late glacial periods. These intervals are separated by a brief deglacial period of apparently higher sedimentation rates and 230Th deposition. During the Holocene, the average sedimentary inventory of 230Th at these sites is largely within 30% of the water column production, in good agreement with observations and model results from other ocean basins.

231Pa and 230Th in surface sediments of the Arctic Ocean: Implications for 231Pa/ 230Th fractionation, boundary scavenging, and advective export

Activities of 231Pa and 230Th in surface sediments from the Arctic Ocean and Greenland–Norwegian Seas are used to examine the redistribution of these water-column produced tracers between the low productivity interior basins and high particle flux marginal areas. Sediment 231Pa xs/ 230Th xs ratios throughout the Canadian and Eurasian Basins and the high particle flux Chukchi slope region are essentially invariant (average = 0.063 ± 0.014, N = 39) and significantly below the water column production ratio (0.093). 231Pa xs/ 230Th xs ratios are also below the production ratio in the seasonally high productivity Greenland–Norwegian Sea basins (average = 0.082 ± 0.024, N = 13), though they are ∼ 30% higher than the interior Arctic. The low sediment 231Pa xs/ 230Th xs ratios are attributed to the net export of ∼ 39% of dissolved 231Pa produced in intermediate/deep waters, as opposed to ∼ 10% of 230Th, through Fram Strait. The negligible 230Th export, combined with the reported low 230Th xs inventory of basin sediments and high 230Th xs inventory in Chukchi slope sediments, points to significant boundary scavenging of 230Th in the Arctic. The invariant 231Pa xs/ 230Th xs ratios further indicate that scavenging of 230Th is comparable to 231Pa (i.e., fractionation to a similar degree between the two nuclides) in both the interior basins and margins. Thus, while significant boundary scavenging of both 231Pa and 230Th occurs in the Arctic, there is a distinct lack of basin-margin fractionation and a sizable export of 231Pa. There is also likely to be export of other similarly particle-reactive radionuclides (e.g., 10Be, Pu isotopes) out of the Arctic.

210Pb sedimentation rates from the Northwestern Mediterranean margin

A total of 83 cores were collected in the Gulf of Lions continental margins and analysed for 210Pb xs (excess 210Pb) in order to understand sedimentation patterns. Apparent sedimentation rates (ASR) range from 0.65 cm year −1 in the vicinity of the Rhône River mouth to 0.01 cm year −1 in the deep basin. Except for the prodelta area, rates decrease with depth linearly with the water depth. On the slope, ASR do not differ between canyons and open slope, except for the western area where the rates are slightly higher in the Lacaze–Duthiers canyon compared to its adjacent, open slope. In the canyon and open slope areas, mass accumulation rates determined from 210Pb xs profiles (0.10 and 0.08 g cm −2 year −1, respectively) are in good agreement with particulate fluxes calculated from 5 years of near-bottom sediment trap data, even when the trap particle fluxes and the apparent accumulation rates are overestimated in response to resuspension and bioturbation effects. However, differences in sediment trap data, between west and east portion of the slope, are not observed in the sedimentation rates calculated with 210Pb xs. The outer shelf area may have an important role in trapping sediment but it is not sufficiently documented. Sediment surface mixed layer depths decrease with water depth, with a mean value for the whole margin of 8±6 cm. 210Pb xs inventories in the sediment are systematically higher than the net 210Pb export flux estimated from the above water column. Over the margin, the ratio between accumulated 210Pb and available 210Pb is about 3, suggesting boundary scavenging effects and advective transport.

Scavenging of 230Th, 231Pa and 10Be in the Southern Ocean (SW Pacific sector): the importance of particle flux, particle composition and advection

Abstract The scavenging of 230Th, 231Pa and 10Be was studied in the Pacific sector of the Southern Ocean along 170°W using measurements from surface sediments, time-series sediment traps, and the water column. All sediment traps collected an annual flux of 231Pa greater than the integrated rate of production by 235U-decay (up to 5.5 times greater) and a flux of 10Be greater than the global-average deposition rate of 10Be (up to 3.4 times greater). Fluxes of 230Th were on average close to the production rate in the overlying water column. These large fluxes of 231Pa and 10Be, and high unsupported 231Pa/231Th and 10Be/230Th ratios in the sediments, are not associated with depletion of nuclides in the water column, and therefore are not the result of the classical boundary scavenging mechanism. We estimate that of the 231Pa and 230Th advected into the Southern Ocean as part of the large-scale overturning circulation, only those nuclides associated with the “upper limb” of this circulation, i.e. those that pass through the surface as part of the wind-driven Ekman flow, are scavenged efficiently. The majority of the nuclides advected into the Southern Ocean and associated with the “bottom water limb” are not scavenged to the sediments of the Southern Ocean, but are returned northward. Throughout the year, the unsupported 231Pa/230Th and 10Be/230Th ratios of material reaching sediment traps at ∼1000 m at a given site increase with increasing particle flux. This behavior is explained by a conceptual model in which Th, Pa and Be are scavenged reversibly by fine-grained suspended particles at all depths, while Pa and Be are in addition scavenged strongly by diatom-rich aggregates in surface waters. Spatial variability in the annually averaged unsupported 231Pa/230Th and 10Be/230Th ratios of sinking particulate matter reflects primarily the variability in particle composition, and in the ratio and quantity of nuclides upwelled to the mixed layer, rather than variability in particle flux.

Beryllium-10 in deep-sea sediments: a tracer for the Earth's magnetic field intensity during the last 200,000 years

We present a new approach for the reconstruction of the Earth's magnetic field intensity during the last 200,000 years based on 10Be in deep-sea sediments. A simple box model that is able to describe boundary scavenging of 10Be in the ocean is used to study the sensitivity of 10Be to changes of water circulation and/or varying particle rain rates. The model is applied to separate transport and production signal in single 10Be records. Vertical sediment accumulation rates are used as input values for the model calculations. The calculated scavenging factors are applied to correct the 10Be records for boundary scavenging. The model corrected 10Be based paleointensity record correlates well with other records from very different archives.

Dissolved and particulate 231Pa and 230Th in the Atlantic Ocean: constraints on intermediate/deep water age, boundary scavenging, and 231Pa/230Th fractionation

231Pa and 230Th concentrations were determined in filtered seawater and suspended particulate matter collected from the Labrador Sea and the Equatorial and South Atlantic to constrain their application as tracers of intermediate/deep water age and Atlantic thermohaline circulation. Distributions of total 231Pa and 230Th indicate the influence of recently formed North Atlantic Deep Water, as evidenced by nearly invariant concentrations below ∼1000 m in the Labrador Sea and increasing 231Pa and 230Th concentrations as deep waters progress southward from northern source regions. Application of a scavenging–mixing model to both tracer distributions indicates an intermediate/deep water age of 12 yr in the Labrador Sea and a ∼30–140 yr transit time to the low-latitude stations. We attribute a striking increase in total 230Th in the Labrador Sea from 1993 to 1999 to aging of intermediate waters as a consequence of the cessation of deep convection in the Labrador Sea since 1993. The temporal change in the 230Th age of these waters is consistent with the 6 yr time interval between the observations. The average particulate 231Pa/230Th activity ratio in the Labrador Sea and low-latitude deep waters is 0.057±0.003, significantly below the 231Pa/230Th production ratio (0.093) and in agreement with excess 231Pa/230Th ratios in Holocene sediments (0.060±0.004) and sediment trap material (0.034±0.012) from the Atlantic and model simulations. This observation is consistent with the southward transport of deep water strongly attenuating boundary scavenging in the Atlantic. A latitudinal dependence in particle fractionation of these tracers is also evident, with elevated fractionation factors (FTh/Pa) observed near the Equator and South Atlantic gyre (∼11) compared to low values in the Labrador Sea (∼3) and Southern Ocean (∼2). There also exists a depth dependence in FTh/Pa, characterized by low values in surface waters, a broad mid-depth maximum, and decreasing values towards the sea-floor. The latitudinal and depth variations in FTh/Pa are suggested to reflect differences in the chemical composition of marine particles.